1. Field of the Invention
This invention relates to a method of manufacturing an optical semiconductor element, and more particularly to a method of manufacturing an optical semiconductor element having a ridge waveguide.
2. Background Art
DVD (Digital Versatile Disc) recording systems are widely used for television broadcasting and computer high-capacity recording. Writing a conventional DVD requires a high-power semiconductor laser device in the 730-nanometer wavelength band.
In order to achieve such a high-power semiconductor laser device InGaAIP-based semiconductor laser elements of the so-called “ridge waveguide type” are used. This structure has a striped or tapered ridge portion formed on a cladding layer above a double heterojunction to control horizontal transverse modes, and is also referred to as the “refractive index waveguide structure”.
The process of forming a ridge stripe includes a step of etching away a protection film formed on the sidewall of the ridge portion, and a subsequent step of etching away the InGaAIP cladding layer constituting the ridge stripe. In a conventional method in widespread use, the protection film is removed by dry etching, and then the cladding layer constituting the ridge stripe is etched away by wet etching with phosphoric acid H3PO4 solution.
However, wet etching with phosphoric acid H3PO4 solution tends to spread the base of the ridge portion, and thus the shape of the ridge portion is not sufficiently controlled. When the base of the ridge portion spreads out, higher order transverse modes are likely to occur. The resulting kink generation in the output characteristics is an obstacle to increasing the output power of semiconductor laser devices.
In a disclosed technique (e.g., see JP2003-332691A), a ridge stripe portion composed of a plurality of layers is formed in order to make vertical the slope of the side face of the ridge portion and to increase the ridge height.
However, this technique has a problem of increasing the number of steps for forming the plurality of layers, which deteriorates manufacturing productivity.
More specifically, the poor selection ratio between the etched cladding layer and the underlying etch stop layer makes the etching amount less controllable. Therefore the slope of the upper side face of the ridge portion is difficult to be made vertical, and a base portion occurs on the lower side face of the ridge portion.
In addition to semiconductor laser elements, the side face of the ridge is often required to be made vertical in ridge waveguide type LEDs (light emitting diodes), light receiving elements, optical waveguide elements, optical switching elements, and filter elements.